Electroless nickel plating bath control



United States, Patent "Ce ELECTROLESS NICKEL PLAT-ING BATH CONTROLRobert A. Spanlding, Huntington Woods, Mich., assignor to General MotorsCorporation, Detroit, Mich., a corporation of Delaware I No Drawing.Filed July 3, 1953, Ser. No. 366,069

1 Claims. 01. 106-1) This invention relates to improvements in thedeposition of nickel from chemical reduction plating solutions and moreparticularly to an improved plating bath, its control and maintenance.

It is known that nickel can be deposited on certain types of metals byimmersion of the metal in an aqueous solution of a nickel salt and areducing agent such as sodium hypophosphite or potassium hypophosphite.There .will be'seen that continued operation of this type plating bathresults in an undesirable lowering of the pH by 7 the accumulation ofhydrochloric acid.

- Accordingly,-for continuous or extended plating operations it isnecessary, in addition to replenishing the nickel and reducing agent asthey are used up, to regulate the .bathpH" by additions of an alkali. Inthe past, this has been accomplished by additions of a water solublenickel salt such'as nickel chloride, an alkali such as sodium hydroxide,and a reducing agent, generally sodium hypophosphitea However, priorreduction plating bath replenishment techniques, in" additiontorequiringextremely accurate pH determinations, involve certaindifiiculties because of the by-products formed during'the interreactionof the .added ingredients. For example, in a typical chemical reductionplating bath, utilizing nickel chloride, which has been in use forsometime there are quantities of hydrochloric acid, sodium dihydrogenphosphite and/ or phosphate, and lesser amounts of'reducing agent suchas sodium hypophosphite, as well as the dissolved nickel .salt. Ifanother water-soluble nickel salt such as nickel sulfate is used, thecorresponding acid is formed as the nickelis depleted. When caustic sodais added to such a bathto adjust its pH, and awater-soluble nickel saltsuch as'nickel chloride is added to replenish the nickel content, thereoccurs an interreaction of the sodium hy- 1 d1'0Xid and nickel chlorideand/ or the hydrochloric acid already present to form sodium chloride orother undesirable byproducts 'which. accumulate and necessitate disposal of the plating solution. t

I have now discovered that the above-mentioned difii culties can beavoided andthe operating life of a chemical reduction plating bath canbe greatly extended by utilizing a water-insoluble nickel'salt as asource of nickel. More particularly, the water-insoluble nickel saltscontemplated in the present invention are those adapted, when PatentedOct. 1-1, 1960 2 v dissolved in the plating solution, to regulate thebath pH, thus eliminating thenecessity of separate additions of alkali.

Examples of suitable water-insoluble nickel salts are: nickel oxy saltssuch as nickel monoxide, nickel peroxide, nickel sesquioxide, nickelousnickelic oxide, and nickel superoxide; hydroxy nickel salts such asnickelic hydroxide and nickelous hydroxide; and carbonates such asnickel carbonate andbasic nickel carbonate. By employing one or' more ofthese water-insoluble nickel salts as a source of nickel, either inreplenishing a conventional chemical reduction bath or in the-originalbath makeup, not only is the desired nickel content provided, but alsothe bath pH is regulated by 'the alkaline radical liberated in theplating solution. e 1

Typical of a nickel reduction plating bath which may be regulated inoperation by the practice of the present invention is the followingcomposition where the quantities expressed are per liter of water:

Nickel chloride "grams" 30 Sodium hypophosphite do 10 Glycollic acidmilliliters; 35 Sodium hydroxide "grams" 15 K It will. be understood ofcourse that the above bath composition is intended only as anillustration of a typical reduction plating bath and that the presentinvention is applicable to a wide range of bath compositions. Forexample, plating bathscontaining about 5 to 50 grams per liter of aWater-soluble nickel salt such as nickel chloride, nickel acetate,nickel sulfate, etc.; approximately 5 to.100 grams per liter of areducing agent, such as hypophosphorous acid, sodium hypophosphite orother soluble hypophosphites; and about 15 to grams per liter of abulfersuch as glycollic acid, sodium hydroxy acetate, sodium acetate, sodiumcitrate, etc. and/ or other bath. additives can vsimilarly bereplenished in accordance with the present invention.

Although various chemical reactions occur during the plating processrandthe bath replenishment, the exact interdependence of which is notclearly understood atpre sout, the following reactionsrillustrategenerally how. the nickel salts of the present invention servetojreplenish the nickel content andregulate the bath pH.,

Employing nickel hydroxide:

Using basic nickel carbonate; 1 V V ZNiCO 3Ni( OH) 2 4H O+ l0HCl 5NiCl+9H O+CO As shown in these reactionis,when a nickel salt of the toregulate the pH.

By employing water-insoluble nickel salts, the necessity of makingfrequent bath analyses for nickel content is eliminated. Moreover, theplating solution is not contaminated with by-product sodium chloride orother compounds, and since no foreign alkali need be added to the bath"to regulate its pH, accurate pH determinations are no longer necessary.

While 'tlie'pre'sent invention has thus faribeen described in somedetail as being applicable for the replenishment and pH control ofconventional reductionplating baths, it is to be understood that thepresent invention also contemplates the formation o f animprovedchemical reduction plating bath utilizing awater-insoluble nickel-saltas an initial ingredient. Such a bath comprisesan aqueous mixture of areducing agent and a water-insoluble nickel salt selected from the groupconsisting of oxy nickel salts, hydroxynickel salts and nickelcarbonates. In most intanc s i e i b eto ad i io a y emp y. a. bufferrgagent, ,A preferred bath of this 'type comprisesnickel car*bonate,"sdium hypophosphite, and "gl'ycollic acid.

A bath of the above-mentioned 'type can be produced 'acetate,'sodiumcitrate or sodium hydroxy acetate, the mckel salt may first be dissolvedin hypophosphorus or phosphorus acid, enough acid being U ed to obtainthe desiredinitial bath pH.

The following is an example of one reduction plating bath utilizing awater-insoluble nickel salt as'an initial ingredient The quantitiesexpressed are per liter of water.

n A satisfactory range of ingredients comprises a mixture of about 5 to100 grams per liter of a water-insoluble nickel salt, approximately '15to 100 g n the bath consisting ofa mixture of. insoluble nickelcompound, a hypophosphite reducing agent and'bulfer, if desired, may beused. The practice of the present invention providesconvenient means:for prolonging the operating life of a conventional type electrolessnickel plating solution. Moreover, the invention also provides animproved plating solution in which an undesirable foreign ion need 7 pdissolving the desired water-insoluble. nickel salt or salts inglycollie acid and thereafter adding thereducing agent. If another typebuffer is used, for example, sodium not beintroduced and in which theproduction of an undesirable lay-product is eliminated..., j 1.

It is to be understood-that, although the invention has been-describedwith specific reference to particular em- 'bodiments' thereof, it is notto be'so limited since changes per liter of "a bufier such as glycollicacid, sodium hydroxy' acetate, 7

sodium acetate, sodium citrate, etc., and about 5 'to 100 phosphite,hypophosphorons'acid, or other hypophosphites.

7 Since the nickels'alts of the present inventionare e x-. tremelyinsoluble. in water,-eve n at elevated temperatures, 7

' the use of. these salts in the replenishment of conventional reductionnickel plating baths involves certain difficulties grams per liter ofreducing agent such assodium hypoand alterations therein maybe madewhich are within'the full intendedscope of this invention as. defined bythe appended claims. 1T

Whatisclaimed isr p p 7 1. In a chemical reduction plating processutilizing a plating bath comprising an aqueous solution of awatersoluble nickel .saltand' a hypophosphite reducing agent, theimprovement which consists in maintaining the desired nickelconcentration in the bath by aclditions' of a waterinsoluble nickelcompound ,which'is' adapted to-increase the pH of thefplating bath. v

2. In a chemical reduction nickel plating-process of the type utilizingajplat'ing bath comprising an aqueous solu- "tion of a water-solublenickel salt and a soluble hypophos phite reducing agent, the improvementwhich consists-of adding to the bath at least one water-insoluble nickelcompoundselected. f om the group consisting of nickel oxide, nickelhydroxide'nickel' carbonate and basic nickel carbonate. H V v 3. Amethod of maintaining the pH'and desired nickel concentration in achemical reduction nickelplating-bath containing a soluble nickel saltand. a soluble hypophoswhich I have succeeded, iii-overcoming by thefollowing .form of a slurry by stirring thefinely divided nickel saltdivided water-insoluble nickel salt as by introducing the slurry intothefilter intake. The'used plating solution is then passed. through-thesame filter-whereby a quantity 'of the insoluble nickel salt isdissolved into the plating fbath, In practice, "the amount dissolvedfrom the filter method:.A water-insoluble nickel salt is employed inthe.

phite reducing .agenbeaid method comprising the steps of preparing awater slurry of a water-insoluble nickel compound, depositing a coatingof said slurry on a filter,

' and thereafter passing said nickel-plating bath'through I into water.The filter conventionally employed. in chemical reduction platingsystems is then coated with the finely inedia will beapproximatelyequivalent to the amount of a nickel consumed in theplating. Hence,'the plating bath {will constantly replenish itself untilthe charge of the insoluble nickel salt hasrbeen consumed -Moreover,since I j the amount of nickel consumed in vthe reduction plating 7operation correspondsto "theamount of acid produced,

, V the'alkalineradical liberated in solutionby the gradual dissolvingof the nickel saltwill neutralize an equivalent ainountofflacid. Thusthe pH will remain relatively constant.

In general, electroless nickel reduction plating:baths of I the typecontemplated herein operate most satisfactorily at a pH within the rangefrom about 3.5 ton pH at which nickel hydroxide is precipitated. Apreferred'pH range is from 5.0 to 5.5..

Plating baths embodying the'present invention generally I should beoperated at a relatively high temperature for" optimum results. I have'found'that the bath tempera; 'ture'in most instances "should'be. atleast 160? F. although a temperature Within the range from abouti190.:F.- -210 .Fis'preferredf said filter to dissolve it erenefi an;waterinsoluble nickel compound equivalent to the nickel consume'd inplating.

4: In a chemicalreduction nickelaplating processiofl the type utiliiingan aqueous plating solution comprising about 5 to 50 grams per liter ofawater soluble nickel salt, approximately 5 to 1 00 gramsper literofahypophosphite reducing agent,- and about 1'5 to 100 grams per liter'ofa butter, the improvement consists et replenishing the nickelcontent ofsaid plating solution as it is depleted during plating ;by addition ofatleastone water-insoluble nickel compound selected fromthe groupconsisting of nickel oxide, nickel hydroxidegniekel carbonate, and basicnickel carbonate. 7 1

5. A chemical reduction type n1 kel plating bath-com prising anaqueoussolutionofanickei salt, a hypophosphite reducing agen't andaQbufferQsaid solution being in contact with a water-insolublenickebcoinpou' ndi so ihatf acid generated in said bath during platingwill react therewith to increase the-pl-I and replenish the solublenickel salt.

6; A chemical reduction type nickeliplat ng bath comprising an aqueoussolution of. a 'solublegmckel salt, a

. hypophosphite reducing agent and'abufier, saidsolution being incontact witha nickel compound selected from the 7 group consisting ornickel oxide, nickel'hydroxide, nickel .Fromthe,foregoing description itwill'now be understood'that'I have provided a new and different typechmical reduction nickel plating operation which the nickel #saltisreplenished'and the pH LregulatedbY'adEiing to'the 7 group consisting ofnickel oxide-nickel hydroxide; nic'kl' carbonate, andbasic nickelcarbonate. -Alsti, ini-accordanee withthe inventiom-an additiyefreplenish or bath atperiodic intervals; or'continuously in certainappli-- 1 cations, -a wateninsoluble nickel; salti selected from theIQarb'onateTand the basic nickel nbe ate;

a 7 7. In a chemical reduction nickel plating proe'ess'ofthe *typeutilizing 'aplating bath comprising an'faqueous soln- *tion" ofa "watersoluble nickel"salt,"a'ireduc1ng agent 7 'selected'fror'n the groupconsisting of s'odiumliypophosphitefand potassiumhypophosphite, anda3butler,+sa1d bath being'maintairied at'ia pHiof about ,trom-3 .5 to-6,4)'fand; at a temperature of at least- F5, the improvement which Iconsists of regulating the 'pH and the desired nickel content of thebath during operation'thereof bygtheaddition Qf?": .iikl"compoundselected from the; groupconsisting an. n. I

of nickel oxide, nickel hydroxide, nickel carbonate, and basic nickelcarbonate.

References Cited in the file of this patent UNITED STATES PATENTS 5 Dowet a1. Sept. 12, 1911 Roehl et a1. Feb. 13, 1951 Talmey et a1 Nov. 10,1953 Gutzeit et a1 Nov. 10, 1953 10 6 Gutzeit et a1 Nov. 10, 1953Spaulding Dec. 13, 1955 OTHER REFERENCES Blum et 211.: Principles ofElectroplating and Electroforming, 3rd edition, 1949, pages 363-364.

Hodgman et al.: Handbook of Chemistry and Physics," Chemical RubberPublishing Co., Cleveland, 26th ed. 1942), p. 416.

1. IN A CHEMICAL REDUCTION PLATING PROCESS UTILIZING A PLATING BATHCOMPRISING AN AQUEOUS SOLUTION OF A WATERSOLUBLE NICKEL SALT AND AHYPOPHOSPHITE REDUCING AGENT, THE IMPROVEMENT WHICH COMSISTS INMAINTAINING THE DESIRED NICKEL CONCENTRATION IN THE BATH BY ADDITIONS OFA WATERINSOLUBLE NICKEL COMPOUND WHICH IS ADAPTED TO INCREASE THE PH OFTHE PLATING BATH.